The invention relates to a process for the fractionation of oil and gas on a petroleum deposit effluent.
The treatment of a petroleum deposit effluent usually involves a stage for oil-gas fractionation which aims to adjust the bubble point of the oil produced in such a way that it is rendered thermodynamically stable under storage conditions.
The treatment can also involve a de-acidification stage for the oil produced which consists mainly in extracting most of the hydrogen sulphide(H.sub.2 S) for toxicity and corrosion related reasons.
These stages are usually simultaneous, with the effluent produced undergoing a series of successive expansions, usually 3 or 4, from the pressure at which the effluent issues from the well to a pressure which is close to atmospheric pressure, the gas produced being removed or recompressed with each expansion, the effluent usually being heated prior to the last expansion in order to satisfy vapor pressure and H.sub.2 S content requirements.
The specified requirements for the oil produced are as follows:
For the vapor pressure, the criterion usually taken is the vapor pressure at 100.degree. F. expressed as PSI or Reid Vapor Pressure (RVP). This specified requirement is usually between 8 and 12 PSI.
For the H.sub.2 S content, the usual specified requirement is 60 ppm. mass.
The gas produced on each expansion contains heavy constituents (C.sub.4.sup.+) which do not have to be withdrawn from the oil in order to stabilize it. It is desirable to recover these heavy constituents so that they can be reinjected into the oil, since this increases the amount of stabilized oil produced, while reducing its density, which increases its marketing value. Moreover, the presence of these heavy constituents in the gas produced confers upon the gas a high hydrocarbon dew point which can have adverse effects upon marketing the gas.
Recompression of the gas produced on each expansion and also partial condensation thereof by cooling makes it possible for a part of the heavy constituents to be recovered, but also makes for an increase in extra investment costs with the compression output needed.